This invention relates to an antenna device which combines an antenna and a filter into a single unit.
As shown in FIG. 5, a prior art antenna device used for a system such as the GPS (Global Positioning System) installed on an automobile is composed of an antenna, a filter, a preamplifier and a receiver. Signals received by the antenna are sent to the filter, by which a desired frequency is selected, and are transmitted to the receiver through the preamplifier. As shown in FIG. 6, such a prior art antenna device 57 is formed by providing a printed circuit board 51 with electrode patterns 51a and 51b formed on its surfaces and mounting a microstrip-type antenna 55 on its upper surface and a filter 56 on its lower surface, the antenna 55 having a radiating electrode 53 formed on the upper surface of a dielectric substrate 52 and a grounding electrode 54 formed on the lower surface of the dielectric substrate 52. A feed line 58 is provided to connect a feed point 53a on the radiating electrode 53 of the antenna 55 to one end of the electrode pattern 51a on the lower surface of the printed circuit board 51, and the filter 56 is attached to the opposite end of the electrode pattern 51a. Thus, electromagnetic waves received by the radiating electrode 53 of the antenna 55 are transmitted to the filter 56 through the feed line 58 and the electrode pattern 51a. The grounding electrode 54 of the antenna 55 is connected to the ground through the other electrode pattern 51b on the printed circuit board 51.
For a prior art antenna device of this type, however, the antenna 55 and the filter 56 must be manufactured separately and mounted to the printed circuit board 51 separately. This means not only that both the cost of parts and that of the mounting are high but also that the antenna device 57, as a whole, becomes rather large. Since the antenna 55 and the filter 56 are connected through the electrode pattern 51a, furthermore, the impedance matching between the antenna 55 and the filter 56 becomes unstable. This has the adverse effect of increasing the reflection loss and the transmission loss of the signals.